At the present time, the major bonding systems being used in the manufacture of composite products of wood still utilize resin condensates of formaldehyde with urea or melamine or with phenol or other phenolics. As a result of general economic conditions and especially the cost of petrochemicals, there is a continuous incentive to reduce the cost involved in the manufacture of bonded wood products. In addition, urea-formaldehyde resin releases formaldehyde which is carcinogenic and an eye and mucosa irritant, thus creating a possible health problem, and certainly giving the wood product an undesirable odor
Adhesives based on carbohydrates, such as starches or sugars, which are converted to binders by various means, have also been suggested and have been adopted to some extent. Carbohydrate binders have an important advantage in lower production costs and zero formaldehyde emission. On the other hand, synthetic resin binders based on the condensation of formaldehyde with urea, melamine or phenol have a significant advantage over carbohydrate binders in requiring lower curing temperatures and shorter curing times, which are important production cost affecting factors.
An improved system has recently been developed as set forth in Stofko co-pending application Ser. No. 761,793, and its parent Ser. No. 580,157 filed Feb. 14, 1984, in which mixtures of sugars and formaldehyde resins are utilized, which mixtures have a number of advantages, including cure time and cure temperatures substantially equivalent to those of formaldehyde condensation resins, yet which admixtures are less expensive because of the utilization of substantial quantities of sugar and which release no formaldehyde into the environment.
Isocyanates, especially 4,4'-diphenylmethane diisocyanate, hereinafter DMI, have been suggested as binders for wood. However, wide industrial use of DMI as a wood binder has never materialized because DMI is more expensive than the conventional formaldehyde-based adhesives, such as urea-formaldehyde. At current prices, DMI is five times more expensive than urea-formaldehyde resins, and its use at even the minimum suitable quantity of 2% resin content still makes such use about 56% more expensive than the use of urea-formaldehyde resin at 6%, resin content again based on the weight of the wood. This makes isocyanates too expensive for normal commercial production.
It is technically very difficult if not impossible to uniformly distribute 2%, based on the weight of the wood particles, of liquid isocyanate on such a large surface. For this reason, at the present time the state of the art demands at least 3% of DMI at a very minimum for producing interior particle board, and about 4-5% of DMI for producing exterior particle board; 2.5% DMI is necessary for producing waferboards using large wafers as particles. At 3% DMI content, isocyanates are much more expensive than urea-formaldehyde or phenolformaldehyde resins and therefore are uneconomical. It has been attempted to extend DMI by the use of an inactive carrier, such as bark powder, but this has not been successful.